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Versions: 00 01

Internet Engineering Task Force                               T. Fossati
Internet-Draft                                                 KoanLogic
Intended status: Standards Track                             P. Giacomin
Expires: September 11, 2012                                    Freelance
                                                               S. Loreto
                                                                Ericsson
                                                          March 10, 2012


                  Publish and Monitor Options for CoAP
             draft-fossati-core-publish-monitor-options-01

Abstract

   This memo defines two additional Options for the Constrained
   Application Protocol (CoAP) especially targeted at sleepy sensors:
   Publish and Monitor.

   The Publish Option enables opportunistic updates of a given resource
   state, by temporarily delegating the authority of the Publish'ed
   resource to a Proxy node.  The whole process is driven by the
   (sleepy) origin -- which may actually never need to listen.

   The Monitor Option complements the typical Observe pattern, enabling
   the tracking of a resource hosted by a node sleeping most of the
   time, by taking care of establishing and maintaining an Observe
   relationship with the (sleepy) origin on behalf of the (sleepy)
   client.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 11, 2012.

Copyright Notice




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   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements Language and Motivation . . . . . . . . . . .  3
   2.  Options  . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Publish Option . . . . . . . . . . . . . . . . . . . . . .  4
       2.1.1.  Publishing a Resource  . . . . . . . . . . . . . . . .  4
       2.1.2.  Updating a Resource  . . . . . . . . . . . . . . . . .  5
       2.1.3.  Unpublishing a Resource  . . . . . . . . . . . . . . .  6
       2.1.4.  Value Format . . . . . . . . . . . . . . . . . . . . .  6
       2.1.5.  Discovery  . . . . . . . . . . . . . . . . . . . . . .  6
         2.1.5.1.  Publishing the /.well-known/core Resource  . . . .  6
         2.1.5.2.  Resource Directory . . . . . . . . . . . . . . . .  7
     2.2.  Monitor Option . . . . . . . . . . . . . . . . . . . . . .  7
       2.2.1.  Public Monitor Registration  . . . . . . . . . . . . .  8
       2.2.2.  Monitor De-registration  . . . . . . . . . . . . . . .  9
         2.2.2.1.  Explicit De-registration . . . . . . . . . . . . . 10
         2.2.2.2.  Implicit De-registration . . . . . . . . . . . . . 10
       2.2.3.  Resource Refresh . . . . . . . . . . . . . . . . . . . 10
   3.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12











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1.  Introduction

   The proposal described in this memo covers the following use case:

      a node N, which is sleeping most of the time, depends one or more
      resources hosted at another sleepy node M. In cases as such, the
      probability of an empty intersection between their respective wake
      periods is very high, making it hard for the two to synchronize.

   In this scenario, using the basic observe [I-D.ietf-core-observe]
   functionality is not enough, as it could lead to lost state updates
   in case N is offline while M pushes its notifications; further, the
   observation may never bootstrap since its initialization needs both
   client and origin awake at the same time.

   This memo introduces two extensions to the Proxy caching
   functionality that give the Proxy an explicit mediation role in the
   sleepy-to-sleepy CoAP [I-D.ietf-core-coap] communication.

1.1.  Requirements Language and Motivation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   This specification makes use of the following terminology:

   Sleepy Device:  a sensor/actuator (usually battery operated) that
      powers down its radio beyond the normal radio duty cycle in order
      to save energy.

   and tries to provide an in-protocol solution for the requirement REQ3
   stated in [I-D.shelby-core-coap-req]:

       The ability to deal with sleeping nodes.  Devices may be
       powered down at any point in time but periodically "wake up"
       for brief periods of time.


2.  Options

   +-----+----------+---------+--------+--------+---------+
   | No. | C/E      | Name    | Format | Length | Default |
   +-----+----------+---------+--------+--------+---------+
   |  YY | Critical | Publish | uint   | 1 B    | 0x2     |
   |  XX | Critical | Monitor | (none) | 0 B    | (none)  |
   +-----+----------+---------+--------+--------+---------+




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2.1.  Publish Option

   The Publish Option enables the sleepy origin to temporarily (i.e. for
   a specified "lease" time) delegate the authority of one of its hosted
   resources to a Proxy node that will start to behave as the origin for
   the Publish'ed resource.  This allows a sleepy sensor to use the
   Proxy as the rendezvous point for one-way sleepy to sleepy signaling.

2.1.1.  Publishing a Resource

   P         S
   |   PUT   | Proxy-URI: coap://sleepy.example.org/res
   |<--------+ Publish: 0110
   |    r    | Content-Type: text/plain
   |         | ETag: 0xabcd
   |         | Max-Age: 1200
   |         |
   |  2.01   |
   +-------->|
   |         |

                                 Figure 1

   The origin server publishes one of its hosted resources, specified by
   the enclosed Proxy-URI, by PUT'ing it to the Proxy with a Publish
   Option attached.  The Publish Option value specifies the CoAP methods
   that clients are allowed to use on the resource (see Section 2.1.4).

   The example in Figure 1 shows a delegation where the GET and PUT
   methods are allowed while POST and DELETE are explicitly prohibited,
   meaning that the resource can be read and updated by clients, but it
   can't be deleted nor created.

   The Proxy, which is voluntarily charged by the resource owner to act
   as the delegated origin for the "lease" time specified by Max-Age,
   replies with a 2.01 if the authority transfer has succeeded.  An
   exact duplicate of the submitted representation is created, and from
   now on it can be accessed using the original URI provided that
   clients go through the delegated Proxy.  If the Publish operation
   does not succeed, the origin transfer fails, and an appropriate
   response code is returned.

   An ETag MAY be supplied as a metadata to be included in responses
   involving the Publish'ed representation.  If no Max-Age is given, a
   default of 3600 seconds MUST be assumed.  The Max-Age value, either
   implicit or explicit, determines the lifetime of the origin
   delegation.  When the Max-Age value is elapsed, the Proxy MUST delete
   the Publish'ed resource value and fall back to its usual proxying



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   function.

   The Publish Option is critical and MUST be present in the request
   only.  If the Proxy does not recognize it, a 4.02 (Bad Option) MUST
   be returned to the client.  If the option value is not correctly
   formatted (see Section 2.1.4), a 4.00 (Bad Request) MUST be returned
   to the client.

   It is sufficient for any client wishing to access the resource to do
   so using the Proxy node that, following the Publish operation, will
   start behaving a the origin, satisfying requests on behalf of the
   sleeping node.

   The Proxy MUST save the identity of the resource Publish'er in order
   to distinguish "maintainance" operations such as update and explicit
   deletion, from "regular" access to the published resource by clients.

   An interesting outcome of this communication strategy is that the
   sleepy origin may really never need to listen on its radio interface.

2.1.2.  Updating a Resource

   P         S
   |   PUT   | Proxy-URI: coap://sleepy.example.org/res
   |<--------+ Publish: 0110
   |    r    | Content-Type: text/plain
   |         | ETag: 0xdcba
   |         | Max-Age: 1200
   |         |
   |  2.04   |
   +-------->|
   |         |

                                 Figure 2

   In order to update the delegated resource state, the sleepy node
   shall send the very same request to the Proxy, which in turn replies
   with a 2.04 (Changed) status code in case the update operation has
   succeeded, or an appropriate error code in case it fails.












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2.1.3.  Unpublishing a Resource

   P          S
   |  DELETE  | Proxy-URI: coap://sleepy.example.org/res
   |<---------+ Publish: 0x0
   |          |
   |          |
   |   2.02   |
   +--------->|
   |          |

   The delegation of a given resource can be explicitly revoked by the
   real origin at any time before the lease time expires, by issuing a
   DELETE request to the Proxy hosting the resource duplicate with a
   Publish Option with value 0x0.

   On successful deletion of the delegation a 2.02 (Deleted) response
   code is returned by the Proxy.

2.1.4.  Value Format

   0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |C R U D 0 0 0 0|
   +-+-+-+-+-+-+-+-+

   Each of the first 4 bits is a flag field indicating whether the
   associated CoAP method (respectively: POST, GET, PUT and DELETE) is
   allowed on the Publish'ed resource.  The remaining 4 bits are unused
   and MUST be set to 0.

   In case the value is missing, the default is assumed to be 0x2, i.e.
   the resource is read-only.

   An all-0 value is used to explicitly revoke the delegation (see
   Section 2.1.3.)

   If the delegated Proxy receives a request with a method that is not
   compatible with the supplied mask, it MUST respond with a 4.05
   (Method Not Allowed) response code.

2.1.5.  Discovery

2.1.5.1.  Publishing the /.well-known/core Resource

   The [I-D.ietf-core-link-format] has no explicit text about "well-
   known" discovery of devices through a Proxy, nor about the
   cacheability rules for such resource.  Even if it seems reasonable to



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   assume that the /.well-known/core URI is both query-able and
   cacheable through a Proxy, on the contrary the situation is not very
   much so.

   In fact, since the "well-known" interface relies on the resource
   origin being implicitly defined by the source address of the UDP
   packet carrying the response, quering the "well-known" interface
   (either unicast or multicast) through a Proxy-URI has little hope to
   be fully functional.  The (ab)use of a an implicit L3 locator as the
   identifier of the resource authority makes "well-known" discovery
   generally incompatible with Proxy mediated communication, unless each
   target URI in a link is given as a URI and not as a relative-ref
   (section 4.1 of [RFC3986]).

   Consequently, in this proposal we assume that the /.well-known/core
   of a sleepy node can be Publish'ed if and only if the target URI in
   the each link is not a relative-ref.

   Its registration is the same as in Figure 1, but the Proxy MAY need
   to treat it in a way that is slightly different from other "normal"
   delegated resources.  In fact, while delegation is in place (i.e. the
   lease period is not elapsed, and neither explicit revocation has
   happened) the Proxy MAY be able to respond to filtered queries
   (section 4.1 of [I-D.ietf-core-link-format]) regarding the Publish'ed
   /.well-known/core.

2.1.5.2.  Resource Directory

   Given the strong requirement on the link formatting given in
   Section 2.1.5.1, it could be preferable (or even necessary) to use
   the Resource Directory [I-D.shelby-core-resource-directory] as a
   means of delegating the discovery of the resources hosted at a sleepy
   node.

   This can be done either by the sleepy node, or automatically by the
   delegated Proxy when a Publish request is received.

   [[Automatic push to RD: check it out]]

2.2.  Monitor Option

   The Monitor Option is a variant of the Observe Option that is aimed
   at solving some issues that may occur when sleepy sensors are
   involved.

   Suppose that the resource of interest is not in cache, and a sleepy
   endpoint wants to Observe it through the Proxy.  If the origin of the
   requested resource is sleeping at the time the observation is



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   requested, the requesting node gets an error, and may need to stay
   awake and retry until the target node gets ready -- which is clearly
   not an option in case the sensor has a very small duty cycle.

   The Monitor Option is used to ask a Proxy to keep a given resource
   fresh by observing it, while the requesting node is sleeping.  Thus
   the sleepy sensor can possibly get the latest representation
   published by the monitored resource when it wakes up, even if the
   origin is sleeping -- and was sleeping at the time the Monitor has
   been requested.

   The Monitor Option is critical and MUST be present in the request
   only.  If the Proxy does not recognize it, a 4.02 (Bad Option) MUST
   be returned to the client.

2.2.1.  Public Monitor Registration

   P        C
   |  POST  | Proxy-URI: coap://sleepy.example.org/res
   |<-------+ Monitor: <empty>
   |        | Max-Age: 86400
   |        | Content-Type: application/json
   |  2.01  |
   +------->| Location-Path: temp
   |        | Location-Path: res
   |        |

                                 Figure 3

   The client POST's the resource to be monitored, identified by the
   Proxy-URI.  The request message contains an empty Monitor Option, and
   possibly specifies a TTL (i.e. an implicit de-registration
   indication) for the monitor through Max-Age.  One or more content
   types for the acceptable representations of the resource are
   optionally specified via the Accept option.  In case no TTL is
   supplied, a default value of 3600 seconds is assumed.

   The operation creates a "monitor" resource at the Proxy, that MUST
   maintain a fresh carbon copy of one or more representations of the
   requested resource depending on the supplied Content-Type's.  For
   convenience, multiple "monitor" resources corresponding to the same
   target resource, can be coalesced into the same monitor object at the
   Proxy -- possibly with the same URI.  In such case, a set containing
   one entry for each registered client is kept, which holds the client
   identities, their expiry and one or more preferred media types for
   their representation(s).  When all entries are deleted (either
   because clients have explicitly deregistered the monitor, or the
   monitor period has expired), the corresponding "monitor" object is



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   deleted.  Note that an underlying cache entry MAY still be kept in
   case the cached representation(s) are still fresh (i.e. the Max-Age
   of the "monitor" resource and Max-Age of the target resource have
   completely different semantics.)

   If the monitor resource is successfully created, the server MUST
   return a 2.01 response containing one or more Location-Path and/or
   Location-Query Options to identify the monitored resource instance,
   which can be used from now on by the requester as an alias to the
   target resource.

   At a later time, the client wakes up and wants to access the
   monitored resource.  It does so by requesting the Proxy monitor
   resource that has been previously created.

   P        C
   |   GET  | URI-Path: temp
   |<-------+ URI-Path: res
   |        | Accept: application/json
   |        |
   |  2.05  |
   +------->| (Content)
   |        |
   |        |

                                 Figure 4

   In case the observation on the target node has not been started
   because the Proxy has not yet been able to contact the origin, the
   Proxy will return a [TBD] error code.

   In case the requested resource was not present on the origin, the
   Proxy will return an empty response (i.e. one with no payload.)

   [[XXX: add an explicit response code perhaps like HTTP 204 ?]]

   In case the monitor resource is not found in the Proxy, either
   because the Proxy has rebooted and lost its state, or the monitor
   resource has been de-registered (see Section 2.2.2), a 4.04 response
   code is returned to the client -- that can recreate it, if needed.

2.2.2.  Monitor De-registration

   The monitor object MUST be deleted at the Proxy when all its
   associated resources have been de-registered or have expired.

   In order to save storage, a Proxy MAY decide to delete a monitor
   resource in case it has not been requested for a sufficiently long



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   time, or for any other reason.  Note that the Proxy may also reboot
   and lose its state, including the state associated to any monitored
   resource.  The requester can realize that the state at the Proxy has
   been lost, and re-instantiate the monitor, when it receives an
   unexpected 4.04 from the "monitor" resource.

2.2.2.1.  Explicit De-registration

   P        C
   | DELETE | Path: temp
   |<-------+ Path: res
   |        |
   |  2.02  |
   +------->|
   |        |

                                 Figure 5

   Explicit de-registration is performed by a client, with a DELETE on
   the URI returned by the Proxy on the corresponding registration.

2.2.2.2.  Implicit De-registration

   Implicit de-registration MUST occur when the monitoring period
   specified by the client via Max-Age expires.  If no Max-Age was
   supplied at registration time, a default of 3600 seconds MUST be
   assumed.

2.2.3.  Resource Refresh

   In order to minimize the number of messages used by the monitoring
   process, the Proxy MUST try to install an observation on the
   requested resource.  In case this first attempt fails, the Proxy MAY
   fall back to repeated poll whose duration is upper bounded by the
   Max-Age value indicated by the client during registration.

   Usual cache validation MUST be applied to the cached copy of the
   monitored resource.


3.  Acknowledgements

   Bruce Nordman.


4.  IANA Considerations

   The following entries are added to the CoAP Option Numbers registry:



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   .------------------------------.
   | Number | Name    | Reference |
   :--------:---------:-----------:
   |  2n+1  | Publish | RFC XXXX  |
   +--------+---------+-----------+
   |  2m+1  | Monitor | RFC XXXX  |
   `------------------------------'


5.  Security Considerations

   Threat:  cache poisoning.
   Countermeasure:  authenticate sender.

   Threat:  unauthorized de-registration
   Countermeasure:  authenticate requester.

   Threat:  Proxy resources' exhaustion.
   Countermeasure:  authenticate requester + quota limit.

   Threat:  global state loss.
   Countermeasure:  cache redundancy.


6.  References

6.1.  Normative References

   [I-D.ietf-core-coap]
              Frank, B., Bormann, C., Hartke, K., and Z. Shelby,
              "Constrained Application Protocol (CoAP)",
              draft-ietf-core-coap-08 (work in progress), October 2011.

   [I-D.ietf-core-link-format]
              Shelby, Z., "CoRE Link Format",
              draft-ietf-core-link-format-11 (work in progress),
              January 2012.

   [I-D.ietf-core-observe]
              Hartke, K., "Observing Resources in CoAP",
              draft-ietf-core-observe-04 (work in progress),
              February 2012.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,



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              RFC 3986, January 2005.

6.2.  Informative References

   [I-D.shelby-core-coap-req]
              Shelby, Z., Stuber, M., Sturek, D., Frank, B., and R.
              Kelsey, "CoAP Requirements and Features",
              draft-shelby-core-coap-req-02 (work in progress),
              October 2010.

   [I-D.shelby-core-resource-directory]
              Krco, S. and Z. Shelby, "CoRE Resource Directory",
              draft-shelby-core-resource-directory-02 (work in
              progress), October 2011.


Authors' Addresses

   Thomas Fossati
   KoanLogic
   Via di Sabbiuno, 11/5
   Bologna  40100
   Italy

   Email: tho@koanlogic.com


   Pierpaolo Giacomin
   Freelance

   Email: yrz@anche.no


   Salvatore Loreto
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: salvatore.loreto@ericsson.com











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